This video will show you the step by step of installing the custom firmware into your GT3B Transmitter.
This mod had been tried on Flysky GT3B and Hobbyking GT3B, and should work with the Eurgle version of the GT3B as well, cos essentially, they are all the same radio... :)

Had been wanting to do this quite some time back, and finally had time to sit down and play around with it. :)
You may ask, why do you want to go for the custom firmware?

Because it'll maximize the full potential of the low cost transmitter!

Below are some of the new features that the custom firmware will add on...